In today's rapidly advancing semiconductor manufacturing industry, semiconductor devices are formed on semiconductor wafers and the time required to process a semiconductor wafer at any given stage, is critically important. A reduction in processing time at any particular processing operation means that more wafers can be processed in a fixed time period and this increased manufacturing tool output improves the efficiency of the tool producing a cost savings. Indeed, it is desirable to increase throughput, often expressed as WPH, wafers per hour, for every processing tool used in the fabrication of a semiconductor device as every increase in wafer output per day includes an associated cost savings. Additionally, as the total cycle time for producing a completed semiconductor wafer decreases, so, too, does the cost associated with producing the semiconductor devices that are formed on the substrate.
In optical lithography exposure tools such as steppers and scanners, each individual chip or a block of chips is separately exposed by the tool which then steps to the next chip or block of chips and repeats the exposure process. If the exposure is a flash exposure, the tool is known as a stepper; if the exposure is done by scanning, the tool is known as a scanner. Each exposure produces a pattern on a photoresist film formed on the substrate. It is conventional in semiconductor device manufacture, to provide alignment marks in the first device pattern and to then align the patterns in subsequent device levels to the alignment mark. To insure accuracy and precision in the very critical alignment process, it is important to prevent damage to or destruction of, the alignment marks after they are formed. In particular, it is important to prevent the etching of the oxide layers formed over the alignment marks because overetching of such oxide layers may result in the exposure and attack of the underlying alignment marks. To prevent such etching, the photoresist is advantageously left covering the alignment marks during the etching process. Positive photoresist is typically used in advanced processing, and it is desirable to prevent the exposure of the positive photoresist formed over the alignment marks, thereby assuring that the photoresist remains over the alignment marks after develop and during the subsequent patterning processing such as etching. At other device levels, in contrast, it is desirable to assure that the deposited film, e.g. metal, is removed so as not to obscure the alignment mark. In this scenario, it is desirable to expose the positive photoresist thereby assuring that the photoresist will be removed. Techniques such as WEE, wafer edge exposure, can be used to expose the entire periphery of a wafer.
Layers such as AA (active area), poly, contact, dual gate, RPO-resist protect oxide and via layers are among the layers in which the photoresist must remain intact over the alignment mark to assure that the etching process used at these layers, does not damage or destroy the underlying alignment marks. In positive photoresist systems, this requires the photoresist over the alignment marks to remain unexposed. Typically, the alignment marks are two in number and disposed in a chip or on a scribe line that is adjacent the peripheral edge of a wafer. Using conventional technology, exposure of the photoresist is prevented by a time-consuming image change procedure that occurs when the stepper/scanner optical lithography exposure apparatus is poised to expose the particular chip or block of chips that includes the alignment mark.
FIG. 1 shows wafer 2 exposed by fifty-one “exposure shots” 4, each exposure shot 4 signifying an individual exposure of the associated portion of wafer 2. Each exposure shot 4 represents a reticle pattern being exposed onto the surface of wafer 2 using the same configuration of the optical lithography exposure apparatus, i.e. stepper or scanner. FIG. 2 shows wafer 2 having been exposed by a plurality of identical exposure shots 4 and two dissimilar, truncated exposure shots 6 over areas of wafer 2 that include the alignment marks and which require a time-consuming image change procedure as will be shown in FIGS. 3 and 4.
FIG. 3 shows the blade configuration used to expose standard exposure shot 4, i.e. the configuration of blades 8, 10, 12 and 14 used to project the reticle image 16 consisting of a plurality of die 18, onto wafer 2. In contrast, FIG. 4 shows the configuration of blades 8, 10, 12 and 14 used to expose a truncated portion of reticle image 16 as truncated exposure shot 6 onto wafer 2. It can be seen that blade 8 must be maneuvered relative to the blade configuration shown in FIG. 3, to produce the configuration shown in FIG. 4. Only truncated portion 20 of reticle image 16 is exposed in truncated exposure shot 6 with portion 22 blocked from exposure by blade 8, for example. In a positive photoresist system, the photoresist remains intact over unexposed portion 22 after develop, protecting subjacent structures such as alignment marks, during the subsequent patterning, e.g. etching operation carried out using the patterned photoresist mask formed by developing the exposed pattern.
The blade change takes up valuable process time and it would be advantageous to maintain the integrity of alignment marks formed near the periphery of a wafer by preventing the exposure of that portion of the wafer using a procedure that does not require a time consuming image change procedure as described above. The present invention is directed to such concerns.